This disclosure relates generally to systems for drilling boreholes for the production of hydrocarbons and more particularly to a drilling system having an acoustic measurement-while-drilling (“MWD”) system as part of a bottomhole assembly for measuring acoustic velocities of subsurface formations during drilling of the wellbores. Specifically, this disclosure relates to a method of processing the acoustic measurements using other formation evaluation measurements to guide the processing. An important aspect of this is the use of semblance processing in which the semblance of array seismic data is determined and peak values of the semblance are used to estimate the slowness (inverse of velocity) of acoustic waves.
Drilling processes generate very complex noise environments that affect LWD (Logging While Drilling) borehole acoustic measurements. Noise and uneven BHA (bottom hole assembly) motion occur when drilling and logging through beds of mixed lithology and variable rock mechanical properties, or when penetrating beds at high borehole inclination. Acoustic log semblance processing then becomes difficult and several possible solutions may present themselves to the processing analyst. When using prior art methods, the processing analyst was challenged to discover satisfactory parameter settings for producing high quality semblance results in some difficult while-drilling environments and complicated formations. Because of environmental conditions and drilling dynamics experienced through these formations, the signal-to-noise ratio of the acquired acoustic signal is only moderate, and it is challenging for an acoustic processor not well aware of local geology to set the proper processing parameters required to obtain good results.
The approach of the present disclosure is to build a petrophysical model from other log data, and then compute from this model theoretical values of acoustic compressional and shear slowness. With a petrophysically-derived slowness as a guide, the acoustic wave analyst can now interactively adjust the processing window in frequency space so as to obtain a clearly defined semblance correlogram resulting in the production of a high confidence acoustic slowness. The end result of this technique provides a much improved acoustic slowness to convolve into a complete geophysical interpretation.
For the purposes of the present disclosure, the following definition of “petrophysical properties” is adopted. A petrophysical property includes “lithology” (the type of rock, commonly determined using log measurements such as natural gamma ray, neutron, density, photoelectric, resistivity and NMR), the porosity, water saturation and permeability. The list is not intended to be all-inclusive.